This invention concerns ferroelectric liquid crystals useful for fabrication fast-switching displays with memory properties for the display of digits, symbols, and images.
It is known that ferroelectric liquid crystals can be used to fabricate fast-switching displays with memory properties as disclosed by N. A. Clark, S. T. Lagerwall, Appl. Phys. Lett. 36, 899 (1980).
The number of ferroelectric liquid crystal compounds has expanded greatly in recent times, but as yet there are no pure materials with optimal properties (S. T. Lagerwall, J. Wahl, N. A. Clark, Proceed. SIO Symposium, October 1985, San Diego, USA; J. W. Goodby, Science 231, 350 (1986), F. Dahl, S. T. Lagerwall, Ferroelectrics 58, 215 (1984), D. Demus, H. Zaschke, Flussige Kristalle in Tabellen II (Liquid Crystals in Tables II) Leipzig 1984).
Many ferroelectric liquid crystals are not very stable chemically and thermally, have high melting points, or else their spontaneous polarization is insufficient. Because their dipole moments are too small, and consequently the associated spontaneous polarization is also small, the derivatives of the 2-methyl-butyl group and other chiral branched alkyl chains are unfavorable for practical utilization due to their excessively long switching times.
Other ferroelectric liquid crystals contain the azomethine grouping and, as Schiff bases, are not very stable with respect to heat, moisture, and acids.
Some ferroelectric liquid crystals derived from chiral .alpha.-chlorocarboxylic acids exhibit excessively high melting points (J. W. Goodby et al., Proceed. of 1983 A. C. S. Meeting, Las Vegas, Nev., 1982; Liquid Crystals and Ordered Fluids, Vol 4, F 1).